1. Field of the Invention
The invention relates to a control system for use in a hybrid vehicle including an internal combustion engine and at least one motor-generator as power sources for running the vehicle. More particularly, the invention relates to a control system that starts the internal combustion engine when required vehicle output (or required vehicle output power) becomes equal to or larger than a given start threshold value, and stops the engine when the required vehicle output becomes equal to or smaller than a given stop threshold value.
2. Description of Related Art
A hybrid vehicle including an internal combustion engine and at least one motor-generator as power sources for running the vehicle is known. In a known example of this type of hybrid vehicle, the internal combustion engine is started when the required vehicle output becomes equal to or larger than a given start threshold value, so that the vehicle runs mainly with power of the engine, and the engine is stopped when the required vehicle output becomes equal to or smaller than a given stop threshold value, so that the vehicle runs with power of the motor-generator. Also, during running of the vehicle with power of the internal combustion engine, if the state of charge of a battery is less than a given value, and power required to be generated from the engine is smaller than a given power, the engine is controlled so that power obtained by adding additional or extra power calculated based on the voltage across terminals of the battery to the power required of the engine is generated from the engine (see Japanese Patent Application Publication No. 2011-240757 (JP 2011-240757 A)). Other examples of the above type of hybrid vehicle are described in Japanese Patent Application Publication No. 11-299004 (JP 11-299004 A) and Japanese Patent Application Publication No. 2011-255824 (JP 2011-255824 A).
In the control as described in JP 2011-240757 A, even when a difference between the power required of the engine and the given power is larger than the additional power, the output power of the engine is increased only by the amount of the additional power. Therefore, in this case, the engine may be operated in an operating region having poor operating efficiency.